I figured the reason the documentary was put on ice was because it might feature the making of songs of experience. The announcement for the doc made it sound very in depth, and if the band were working on SoE (they even had a travelling studio) on tour, I'm sure it would be covered during the programme. Maybe they didn't want to show all that off before the album was out.

It also lined up nicely with soe being released, or at least announced, around that date - there were rumors that the album was "ready to go" around the Paris shows/documentary airing date, and if they were going to announce a new album and new shows, coinciding that with a live broadcast and documentary seems logical.

It also seems that the documentary was postponed not too long before it became apparent that soe wouldn't be coming out at the end of 2015.

Could all be coincidence, but it lines up quite well, and off the top of my head I can't think of another good reason for the documentary to have been indefinitely postponed/shelved (other than that it simply wasn't finished in time, and they didn't want to release it all by itself, between tour/album cycles, when it's not 'in promotion' of something).